The present invention generally relates to diagnostic coagulation assays, and specifically, to control samples suitable for use in connection therewith. The invention particularly relates to coagulation controls suitable for both prothrombin time and activated partial thromboplastin time assays.
Coagulation control materials are used in the clinical laboratory for quality control of the prothrombin time (PT) and activated partial thromboplastin time (APTT) assays. PT assays employ thromboplastin reagents and have been used extensively for evaluating blood coagulation associated with the extrinsic pathway. APTT assays employ an intrinsic pathway activator, such as micronized silica, and a phospholipid component of a thromboplastin reagent (without tissue factor protein) for evaluating coagulation associated with the intrinsic pathway. Both PT and APTT assays are used clinically for screening patients' plasma for coagulation factor deficiencies. Clinical screenings are employed, for example, during routine checkups and prior to surgery. PT and APTT assays are also used for monitoring treatment with anticoagulants. For example, PT assays are routinely employed to monitor oral anti-coagulant treatment with coumarin (Warfarin.TM., Coumadin.TM.), and APTT assays are typically used for monitoring anticoagulant treatment with heparin.
Coagulation controls are used for quality control evaluations of the PT and APTT assay systems. The controls are essential in view of potential variation in reagents employed in these assays, potential inaccuracies in the devices used to measure clotting time, and potential effects of inaccurate anticoagulant dosage. Commercial coagulation controls have been designed to mimic three physiologic conditions: (1) "Control Level I" controls mimic normal coagulation and are intended to be representative of an individual without coagulation deficiencies; (2) "Control Level II" controls are intended to mimic the coagulation of an individual undergoing mild anticoagulant therapy; and (3) "Control Level III" controls are intended to mimic the coagulation of an individual undergoing relatively high anticoagulant therapy.
Various types of coagulation controls are known in the art. Typically, coagulation controls are designed to be suitable for use with (1) both the PT and APTT assays, (2) the PT assay only or (3) the APTT assay only. The controls designed exclusively for use in evaluating only the APTT assay are referred to in the art as heparin controls. While effective for APTT assays, such controls are ineffective for use in evaluating PT assay systems. The present invention is directed to general coagulation controls that can be employed for evaluating both PT and APTT assay systems.
A primary performance criteria for a coagulation control is its stability over time. Zucker et al. reported that coagulation controls prepared by buffering plasma specimens with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) and lyophilizing provided stability in the reconstituted control for eight hours at 25.degree. C. Zucker et al., Preparation of Quality Control Specimens for Coagulation, Am. J. Clin. Path. 53:924-927 (1970). Brozovic and co-workers prepared controls from plasma samples of patients on oral anti-coagulant therapy in combination with HEPES, trisodium citrate and aprotinin, and reported stability of 4 hours to 6 hours after reconstitution when stored at 4.degree. C. Brozovic et al., Stability of Freeze-Dried Plasma Prepared from Patients on Oral Anticoagulants, J. Clin. Path., 26:857-863 (1973). U.S. Pat. No. 5,721,140 to Speck et al. discloses a coagulation control comprising normal human plasma, clotting factor-deficient non-primate mammalian plasma and aprotinin, and report that the controls are stable in the absence of a buffer for up to five days. Numerous other patents and literature references describe various coagulation controls. Exemplary patents include U.S. Pat. No. 3,947,378 to Babson, U.S. Pat. No. 4,007,008 to Becker et al., U.S. Pat. No. 4,056,484 to Heimburger et al. and U.S. Pat. No. 4,127,502 to Li Mutti et al.
While many variations in coagulation control compositions have been reported in the art, such controls remain limited with respect to stability--particularly once reconstituted from the lyophilized form in which they are typically sold. Reconstituted commercially-available coagulation controls stored for more than about eight hours do not provide consistent, reproducible clotting times as determined by PT and/or APTT assays. Moreover, precipitates and fibrin have been observed in such reconstituted controls. There remains a need, therefore, for coagulation controls suitable for use in connection with PT and APTT assays that have enhanced stability.